After screening the data base of the Berkeley Drosophila Genome Project with a sequence coding for the transmembrane region of a G protein-coupled receptor, we found that Drosophila might contain a gene coding for a receptor that is structurally related to the Gonadotropin-Releasing Hormone (GnRH) receptors from vertebrates. Using the polymerase chain reaction, with Drosophila cDNA as a template, and oligonucleotide probes coding for the presumed exons of this gene, we were able to clone the cDNA coding for this receptor. The transmembrane region of the receptor shows 36% amino acid residue identity with the transmembrane region of the catfish and 31% amino acid residue identity with that of the rat GnRH receptor. The Drosophila receptor gene contains six introns, whereas the rat gene contains two: one intron in the Drosophila gene occurs at exactly the same position and has the same intron phasing as one intron in the rat gene, suggesting that the Drosophila and mammalian GnRH receptor genes are evolutionarily related. Northern blot analyses show that the Drosophila receptor gene is progressively expressed during larval development with a prominent maximum at the 3rd instar larval stage. Pupae contain low amounts of receptor mRNA, while adult flies contain higher levels, with males having about five times more receptor mRNA than females flies. Southern blot analyses show that Drosophila contains only one copy of the receptor gene, which is located at position 27A2-B1 of chromosome 2. This paper is the first report on the molecular cloning of a member of the GnRH receptor family from invertebrates.